1. Field of the Invention
The present invention relates to an ultraviolet (UV) curable conductive resinous composition, and more specifically to an electrically conductive resinous composition that may be used as an adhesive and/or solder paste, and electronic circuit assemblies using the resin of the present invention.
Typical thermally cured compositions contain large amounts of solvent which must be boiled off before the resin may be cured, thus creating potential ecological problems. Furthermore, thermally cured resins are costly in terms of wasted energy, elimination of large quantities of solvent, and slower production times. Generally, thermal cure processes require a long cure time which makes automation of a manufacturing process difficult. (See U.S. Pat. Nos. 4,481,258; 4,362,263; 4,420,500; and, 4,369,207.)
Conventional commercially available epoxy base conductive adhesives usually require curing at elevated temperatures for about 10 minutes, or at room temperature for extended times usually about 24 hours. Because of the long curing time, these adhesives are not suitable for automated in-line assembly.
UV curable conductive coatings provide extremely fast UV cure rates, which makes automation of electronic circuit assemblies possible. UV curable conductive coatings may also be used to make connections where heat cannot be tolerated, and for repairing heat-sensitive components. (See, for example, U.S. Pat. Nos. 3,957,694; 4,088,801; 4,401,686; 4,362,263; 4,420,541; 4,431,683; 4,369,208; 4,411,980; 4,460,427; 4,469,777.)
2. Description of the Prior Art
Ultraviolet radiation (UV) curing technology has been adopted in manufacturing in order to improve quality, increase productivity, and reduce emissions of volatile organic compounds. UV curable compositions provide high cure speeds and solventless resin systems. Generally, a UV cured composition has a high solids content and is a totally reactive resin that can be applied in a single pass.
There are many potential applications for electrically conductive adhesives in the electronic industry such as chip bonding, electrostatic shielding, corona shielding, repair of printed circuit boards, adhesive work, ignition cable, sheet coatings, the manufacture of electrodes, contacts, terminations, surfaces receptive to plating, hybrid circuit paths, land areas, die attachment, surface mount component attachment (SMT), bonding of electromagnetic interference/radio frequency interference (EMI/RFI), conductive paths, lead attachment, grounding, hybrid circuit assembly, lamination of dissimilar metals and plastics, lid bonding, circuit repair work, and connector attachment.
Conductive adhesives have certain unique properties that make them particularly attractive for use in attaching components to printed circuit boards. One advantage is that these adhesives provide a much simpler assembly procedure, than do wave solder or reflow solder processes.
In spite of these disclosures, there remains a need for an ultraviolet curable conductive adhesive that may also be used as solder, may be suitable for use in automated surface mount technology or electronic circuit fabrication, has low resistivity, adequate lap shear strength, good thermal stability, stable conductivity at high temperature and high humidity, and other properties that allow for a wide range of uses in the electronic industry.